Re: lisp performace question: how efficent are (long) list parameters?

[David Kastrup <dak@gnu.org>]

leo <leo@bella.local> writes:

> hi there
> 
> just a little thing for a lisp beginner. i have written a function:
> 
> (defun first-free-position (positions)
>   "returns the first free position in POSITIONS of all frames"
>   (if positions
>       (let ((list-of-frames (frame-list)))
>         (if (frame-on-position (car positions) list-of-frames)
>             (first-free-position (cdr positions))
>           (car positions)))))
> 
> this function recomputes `(frame-list)' for every recursion.

Recursion is not efficient in Elisp.

> so i thought to set `(frame-list)' to a variable which could be
> carried through as parameter:
> 
> (defun first-free-position (positions all-frames)
>   "returns the first free position in POSITIONS of all-frames."
>   (if positions      
>         (if (frame-on-position (car positions) all-frames)
>             (first-free-position (cdr positions) all-frames)
>           (car positions))))
> 
> called like `(first-free-position position-alist (frame-list))' this
> version computes (frame-list) only once, but the list `all-frames' is
> put every time on the parameter stack.
> 
> so, what is more effiecent?

(defun first-free-position (positions)
  (let ((list-of-frames (frame-list)) frame)
    (while (and (setq frame (pop positions))
                (frame-on-position frame list-of-frames)))
    frame))
  

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum